Process for making durable titanium dioxide pigment in the chloride process without wet treatment

ABSTRACT

The present invention relates to a process for making durable titanium dioxide pigment by vapor phase deposition of surface treatments on the titanium dioxide particle surface by reacting titanium tetrachloride vapor, an oxygen containing gas and aluminum chloride in a plug flow reactor to form a product stream containing titanium dioxide particles; and introducing silicon tetrachloride into the reactor at a point down stream of the point where the titanium tetrachloride and oxygen were contacted and where at least 97% of the titanium tetrachloride has been converted to titanium dioxide or where the reaction temperature is no greater than about 1200° C., and preferably not more than about 1100° C.

BACKGROUND OF THE INVENTION

The present invention relates to the chloride process for the productionof titanium dioxide pigment.

This invention provides a route to a durable grade pigment, without thenecessity of depositing surface treatments on the titanium dioxideparticles by wet treatment.

Typically titanium dioxide particles, produced by either the chloride orthe sulfate process, are processed in one or more wet treatmentoperations to deposit metal oxides on the surface of the pigment inorder to optimize the pigment properties of dispersion, optical spacingor durability. Deposits of aluminum oxide or combinations of aluminumoxide and silicon dioxide, used alone or in combination with otheroxides, are typical constituents of commercial titanium dioxide pigment.Such surface treatments are deposited through precipitation of thedesired metal oxide in a wet chemical reaction. Thus, the base pigment,that is, the titanium dioxide particles produced at the exit point ofthe oxidizer in the chloride process or after calcination in the sulfateprocess, must be washed and processed through one or more wet treatmentsteps. Wet treatment is then followed by washing, drying and grinding toproduce a product suitable for use in for example, exterior coatings andplastics products.

Processes to influence the titanium dioxide crystal formation in theoxidizer of the chloride process were taught in British Patent 689,123,and U.S. Pat. Nos. 3,856,929; 4,124,913; and 5,562,764.

U.S. Pat. No. 3,856,929 teaches that by oxidizing a mixed streamcontaining titanium tetrachloride, a silicon halide and a phosphorushalide, the resulting titanium dioxide product was at least 80% byweight anatase.

U.S. Pat. No. 4,124,913 teaches a process producing rutile titaniumdioxide particles at reduced levels of aluminum chloride concentrationby oxidizing aluminum trichloride simultaneously with the titaniumchloride followed by addition of phosphorous trichloride. Thephosphorous trichloride is added at a point in the oxidizer where atleast 80% of the titanium tetrachloride has been converted to titaniumdioxide.

British Patent 689,123 teaches the oxidation of a mixture of titaniumtetrachloride, aluminum trichloride and silicon tetrachloride where theratio of aluminum oxide to silicon dioxide formed in the oxidation isfrom 3:1 to 1:1 and where the temperature is maintained in the range of1000° C. to 1600° C. By this process it is claimed that 90% of thetitanium dioxide formed is in the rutile crystal, and its particle sizeis about 0.5 microns or less.

U.S. Pat. No. 5,562,764 teaches a process for oxidizing a mixture oftitanium tetrachloride and aluminum trichloride followed by the additionof silicon tetrachloride at a point down stream where the temperature atthe addition point is in the range of 1200 to 1600° C. The inventor inthis patent wanted to enhance pigment gloss and carbon black undertone(CBU) without producing a significant anatase component in the pigmentproduct. Although the product according to this patent contained no morethan 0.7 percent anatase, wet treatment was required to produce adurable and suitably dispersible pigment to meet industry standards.

U.S. Pat. No. 3,219,468 discloses the addition of silicon tetrachlorideat a point removed from the addition point of the aluminum trichloridein a fluidized bed oxidation of titanium tetrachloride. The lateraddition of the silicon tetrachloride results in the production of asoft bed of titanium dioxide particles instead of a hard scale on thewalls of the fluidized bed reactor.

So-called vapor or dry process to deposit surface treatments on thepigment in the oxidation step are taught in U.S. Pat. No. 4,050,951; PCTpublished patent application WO 96/36411; and European Patent 0 032 426.In U.S. Pat. No. 4,050,951 post treatment hydrolysis is taught. Thedisadvantage in this system is that the treatment step is a separatestage in the overall process following oxidization that requires theseparation of base pigment from the oxidation product, then grindingfollowed by hydrolysis at temperatures lower than those temperaturespresent in the oxidizer.

PCT application WO 96/36441 teaches a vapor phase treatment processrequiring that the silicon tetrachloride addition must be made at atemperature of more than 1300° C. This application further teaches thatthe addition of metal halides can be made in any sequence and at anypoint in the reactor.

European Patent 0 032 426 teaches a post treatment of titanium dioxideparticles in a fluid bed reactor. This process requires an activationstep where the titanium dioxide particles are contacted with metalchlorides followed by a hydrolysis to convert residual chlorides tooxides and oxide hydrates.

A common teaching in the art noted above is that the addition of silicontetrachloride to the chlorination reaction is made at a temperature ofat least 1200° C. and at a point relatively close to the point where thetitanium tetrachloride was contacted by oxygen. The common belief at thetime of the present invention was made was that at temperatures 1200° C.or less the rate of silicon tetrachloride conversion was so slow thatpigment product would be contaminated by unreacted silicon tetrachlorideas is taught in the article by D. R. Powers, “Kinetics of SiCl₄Oxidation” published in J. Am. Ceram. Soc, vol. 61, No. 7-8, pp. 295-7(1978). According to this understanding of reaction kinetics, even inthe presence of excess oxygen, addition of silicon tetrachloride attemperatures of less than about 1300° C. would result in unreactedsilicon tetrachloride in the product.

The present invention provides a process for the making of a durablegrade pigment product in the oxidation unit of a chloride processtitanium dioxide plant by adding silicon tetrachloride late in thereaction at a point where the reaction temperature is no greater thanabout 1200° C. and the base pigment is essentially formed. The inventorsof the present process wanted to provide a process to make a durablegrade commercial product of acceptable gloss and CBU but without thecost and additional processing required in the typical wet treatmentoperation.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a process for making durable titaniumdioxide pigment by vapor phase deposition of surface treatments on thetitanium dioxide pigment particle surface, the process comprising thesteps of:

(a) reacting titanium tetrachloride vapor and aluminum chloride and atleast a stoichiometric amount of oxygen in a plug flow reactor to form aproduct stream containing titanium dioxide particles; and

(b) introducing silicon tetrachloride into the reactor at one or morepoints downstream of the point where the titanium tetrachloride andoxygen were contacted and where at least 97% of the titaniumtetrachloride has been converted to titanium dioxide.

Before the introduction of the silicon tetrachloride, it is morepreferred that at least 98% of the titanium tetrachloride has beenconverted to titanium dioxide, and most preferred that at least 99% ofthe titanium tetrachloride has been converted.

It is also preferred that the silicon tetrachloride is introduced in anamount sufficient to provide a silicon dioxide content of a surfacetreated titanium dioxide pigment of about at least 1.2% by weight, andthat the aluminum trichloride is added in an amount sufficient toprovide an aluminum oxide content of a surface treated pigment of atleast about 1% by weight.

Steam or oxygen may be introduced at a point downstream of the pointaddition of the silicon tetrachloride, or steam or oxygen may beintroduced along with the silicon tetrachloride.

The present invention also provides a durable titanium dioxide pigmentwherein at least 95% of the pigment particles are completely covered bya layer formed from a mixture of amorphous aluminum oxide and amorphoussilicon dioxide, the pigment produced by:

(a) reacting titanium tetrachloride vapor and aluminum chloride and atleast a stoichiometric amount of oxygen in a plug flow reactor to form aproduct stream containing titanium dioxide particles; and

(b) introducing silicon tetrachloride into the reactor at one or morepoints downstream of the point where the titanium tetrachloride andoxygen were contacted and where at least 97% of the titaniumtetrachloride has been converted to titanium dioxide.

The preferred composition of the pigment of the present invention isthat the concentration of silicon dioxide is at least about 1.2% of thetotal weight of the pigment and the concentration of the aluminum oxideis at least about 1% of the total weight of the pigment.

The present invention provides durable titanium dioxide pigmentparticles having a surface treatment layer comprising aluminum oxide andsilicon dioxide wherein at least 85% of the pigment particles arecompletely covered by a uniform layer formed from a mixture of amorphousaluminum oxide and amorphous silicon dioxide and wherein the pigmentparticles are free of debris.

The present invention also provides a method to determine a point ofintroduction of silicon tetrachloride into a plug flow reactor for theoxidation of a mixture of titanium tetrachloride and aluminumtrichloride in a gas containing at least the stoichiometric amount ofoxygen to produce titanium dioxide particle having a thin, uniform andcomplete layer of surface oxides comprising a mixture of amorphousaluminum oxide and silicon dioxide, the steps of the process comprising;

(a) determining the temperature in the reactor where not more than about3% of the titanium tetrachloride remains unreacted using

$K = {{\frac{\begin{bmatrix}{{2\left( {{100\%} - u_{{TiC}\; l\; 4}} \right)} +} \\{\varphi \times 100\%}\end{bmatrix}^{2}}{u_{{TiCl}\; 4}\left( {\beta + u_{{TiCl}\; 4}} \right)}\mspace{14mu} {and}\mspace{14mu} T} < {\frac{20733}{{\ln \; K} + 6.391} - 273.15}}$

where

-   μ_(TiCl4)=unreacted TiCl4 (%)-   β=O2 (%) in excess of the stoichiometric amount-   φ=feed Cl2 mole ratio (mol/mol TiCl4), and-   T=temperature (C);    and

(b) introducing the silicon tetrachloride into the reactor where thetemperature is equal to or less than the temperature calculated in stepa.

In addition, the present invention includes a process for making durabletitanium dioxide pigment by vapor phase deposition of surface treatmentson the titanium dioxide pigment particle surface, the process comprisingthe steps of:

(a) reacting titanium tetrachloride vapor, an oxygen containing gas andaluminum chloride in a plug flow reactor to form a product streamcontaining titanium dioxide particles; and

(b) introducing silicon tetrachloride into the reactor at one or morepoints downstream of the point where the titanium tetrachloride andoxygen were contacted and where the reaction temperature is no greaterthan about 1200° C. and more preferred no greater than 1100° C.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIGS. 1 a and 1 b show the pigment particles of the present invention.FIG. 1 a is a micrograph of these particles. FIG. 1 b shows a HREMatomic resolution micrograph of the pigment of the present invention.

FIG. 2 shows a micrograph of a typical wet treated durable gradetitanium dioxide commercial pigment.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for making durable titaniumdioxide pigment by vapor phase deposition of surface treatments on thetitanium dioxide particle surface, the process comprising the steps of:

(a) reacting titanium tetrachloride vapor and aluminum chloride and atleast a stoichiometric amount of oxygen in a plug flow reactor to form aproduct stream containing titanium dioxide particles; and

(b) introducing silicon tetrachloride into the reactor at one or morepoints downstream of the point where the titanium tetrachloride andoxygen were contacted and where at least 97% of the titaniumtetrachloride has been converted (3% unreacted titanium tetrachloride)to titanium dioxide.

The term durable as used herein means a pigment suitable for exteriorarchitectural coatings and automotive refinish or color coat/clear coatOEM finishes. Generally such pigments are characterized in that no morethan about 25% of the pigment dissolves in sulfuric acid in the acidsolubility test as described below, and that silicon dioxide representsat least 1.4 to 2% of the total weight of the pigment.

The composition of the oxide treatment deposited by the process of thepresent invention is a mixture of amorphous aluminum oxide and amorphoussilicon dioxide. The thickness of the treatment layer deposited in thepresent invention is not more than about 4 nm. The pigment is more than99% rutile.

The uniformity of the surface treatment according to the presentinvention may be seen in FIGS. 1 a and 1 b. FIG. 1 a shows aphotomicrograph of pigment particles of the present invention. Thetreatment on the surface of these particles can be seen to be completeand uniform. It is believed that the uniformity and the completeness ofthe surface treatment layer in the present pigments results in acidsolubilities of less that 25% even at silica concentrations of about 1%by weight of the total pigment.

FIG. 1 b shows an atomic resolution HREM of a typical particle productof the present invention. In this Figure the region “P” indicates thetitanium dioxide particle, and the layer “C” is the treatment layer ofamorphous oxides of aluminum and silicon. The uniformity and full extentof coverage of the layer is readily visible.

Analysis of samplings of 1000 particles as described below was used todetermine the fraction of particles treated having full, completesurface coverage. When least 85% of the particles had full, completesurface coverage (FIGS. 1 a, 1 b), the acid solubility of theseparticles was equal to that of durable, commercial grade products.Experience has shown that acid solubility of 25% or less correlates withoutdoor exposure required for commercial grade durable pigments. Thus,acid solubility serves as an accelerated durability test.

Full, complete coverage of the particles means that the entire surfaceof the titanium dioxide particle is covered with the layer of surfacetreatment. The product of the present invention is characterized by thefact that at least 85% of the particles are fully and completely coveredby a layer of surface treatment. This layer is thin and uniform. Thethickness of the layer is about 1 to 4 nm for particles that are about1% by weight aluminum oxide and 1.2% by weight silicon dioxide. Higherconcentrations of the surface treatment are expected to produce thickerlayers, but at equal uniformity. Microscopic analysis of the product ofthe Example has shown that about 80% or more of the pigment particles ofthe present invention have a treatment layer thickness of 1 to 2.5 nm,while in less than about 5% of the pigment particles, the treatmentlayer is about 4 nm thick.

The inventors believe that it is the completeness of surface coverageresulting from the process of the present invention that may providepigment durability meeting industry standards at treatment levels ofabout 2 to 3% of the total pigment weight compared to treatment levelsof about 6% required for typical architectural durable grade titaniumdioxide pigment. In each of the treatment levels noted immediatelyabove, the weight percent shown is the combined concentrations ofaluminum oxide and silicon dioxide.

In comparison to the present invention, wet treatment processes depositsilicon dioxide and aluminum oxide on to the surface of the pigmentparticle by precipitation. Wet treatment processes typically producesilica debris, crystalline aluminum oxide, and irregular particlesurfaces such as shown in FIG. 2. Crystalline oxides typical of wettreatments are not found in the product of the present invention.

Elimination of wet treatment offers an advantage in the overall titaniumdioxide manufacturing process in reducing processing steps. Newtreatment compositions offer the potential to produce pigments havingimproved processing characteristics and properties.

In contrast to pigments produced by wet treatment processes, the pigmentof the present invention is free of debris. This lack of debris maycontribute to improved dispersion and improved performance in coatingsand plastics. The presence of debris may be seen in FIG. 2.

Although pigment durability can be achieved at levels of silicon dioxideof about 1% by weight of the pigment, higher levels of silicon dioxideand of aluminum oxide may be deposited on the surface of a pigment ofthe present process. Also other oxides may be deposited using thepresent process, and the pigment of the present invention may be treatedwith organic treatments as is known by one skilled in this art. Althougha durable pigment is produced by the process of the present inventionwhen at least 85% of the particles have a full, complete surfacecoverage, it is preferred that at least 95% of particles having full,complete surface coverage, and a fraction of about 98% or more is evenmore preferred.

In the present process, titanium tetrachloride is preheated to atemperature of from about 300 to 650° C. and mixed with aluminumtrichloride forming a chloride mix which is fed into a pre-heated streamof oxygen. This chloride mix may contain other metal compounds, exceptsilicon tetrachloride, used in the chloride pigment manufactureincluding compounds of boron, phosphorous, zirconium, and others. Theintroduction of phosphorous compounds into the oxidizer is generallypositioned to control corrosion and may be at some point down stream ofthe point where titanium tetrachloride and aluminum trichloride areintroduced into the reactor.

According to the present invention, it is essential that the aluminumtrichloride be added in advance of and in a location far removed fromthe point where silicon tetrachloride is introduced into the reactor.Thus, the preferred location for the addition of the aluminumtrichloride is in a mixture with the titanium tetrachloride.

In the process of the present invention, oxygen is present as an initialreactant and may also be added with the addition of the silicontetrachloride. Although it is preferred to run the present process withthe oxygen in excess of the amount required to oxidize the chloride mix,the process may be operated with the concentration equal to or less thanthe stoichiometric amount.

The addition of silicon tetrachloride according to the present inventionis made when the conversion of titanium tetrachloride to titaniumdioxide is nearly complete. For example, at least 97% of the titaniumtetrachloride has been converted to titanium dioxide. That is, the pointwhere not more than 3% of the titanium tetrachloride remains unreacted.From their work the inventors have found that the point in the reactorwhere about 3% of the titanium tetrachloride is unreacted, the fractionof particles having full, complete coverage by the surface treatment isabout 85%. At the point in the reactor where about 2% of the titaniumtetrachloride is unreacted, the fraction of particles having full,complete coverage by the surface treatment is about 95%. At the point inthe reactor where about 1% of the titanium tetrachloride is unreacted,the fraction of particles having full, complete coverage by the surfacetreatment is more than about 98%. The corresponding amount of titaniumtetrachloride converted to titanium lo dioxide at these points is atleast 97%, at least 98% and at least 99%, respectively.

When the present process is run as preferred, with at least thestoichiometric amount of oxygen, the addition points for silicontetrachloride may be calculated by the following equations:

$K = {{\frac{\begin{bmatrix}{{2\left( {{100\%} - u_{{TiC}\; l\; 4}} \right)} +} \\{\varphi \times 100\%}\end{bmatrix}^{2}}{u_{{TiCl}\; 4}\left( {\beta + u_{{TiCl}\; 4}} \right)}\mspace{14mu} {and}\mspace{14mu} T} < {\frac{20733}{{\ln \; K} + 6.391} - 273.15}}$

where

-   μ_(TiCl4)=unreacted TiCl4 (%)-   β=O2 (%)-   φ=feed Cl2 mole ratio (mol/mol TiCl4), and-   T=temperature (C)    K is the equilibrium constant for the reaction of the present    process:

TiCl₄+O₂→TiO₂+2Cl₂;

Using this equation, one may calculate the point where the silicontetrachloride is first introduced from the feeds going into the reactor.Excess oxygen, β, is the oxygen in excess of that required to convertthe mixture of titanium tetrachloride and aluminum trichloride fed intothe reactor to their respective oxides (the stoichiometric amount). Thefeed chlorine mole ratio, φ, is the ratio of the moles of chlorine feddivided by the moles of titanium tetrachloride fed to the reactor over afixed period of time, for example, per hour. The percent unreactedtitanium tetrachloride, μ_(TiCl4), is not more than 3% as is required bythe present invention. Using the calculated equilibrium constant, K, onecan then solve for the temperature at the point where silicontetrachloride is first introduced according to the present invention.The point in the reactor where this introduction is made according tothe present invention may be determined using the temperature profile ofthe particular reactor.

This calculation is independent of reactor size and pressure andrequires only knowledge of the feed composition (oxygen, chlorine andtitanium tetrachloride in moles per hour) and the temperature profilefor the reactor. Temperature profiles for a given reactor may bedetermined from well-known thermodynamic and heat transfer principles.

This method of calculating the addition points provide some flexibility,based on the feed mix that may be of importance in designing productfeatures to serve a particular pigment end use application.

The present process for making durable titanium dioxide pigment by vaporphase deposition of surface treatments on the titanium dioxide pigmentparticle surface may also be operated with a mixture of titaniumtetrachloride and aluminum trichloride where the oxygen may be presentin an amount less than the stoichiometric amount. In this case theprocess comprising the steps of:

(a) reacting titanium tetrachloride vapor, an oxygen containing gas andaluminum chloride in a plug flow reactor to form a product streamcontaining titanium dioxide particles; and

(b) introducing silicon tetrachloride into the reactor at one or morepoints downstream of the point where the titanium tetrachloride andoxygen were contacted and where the reaction temperature is no greaterthan about 1200° C.

In this case one would use the reactor temperature profile to locate apoint where the reaction temperature is no greater than about 1200° C.,and preferably no greater than 1100° C. The addition of silicontetrachloride would be made at this point or a point down stream of thiscalculated location. The use of the temperature profile and therequirement that the addition of silicon tetrachloride be made at alocation where the reaction temperature is less than about 1200° C. isuseful in cases where oxygen is present in excess, just equal to or lessthan the stoichiometric amount needed to oxidize the chloride mix.

TEST METHODS

Acid Solubility is determined as the amount of pigment that dissolves inhot concentrated sulfuric acid.

A small sample of pigment was placed in hot sulfuric acid (about 175°C.) and digested for an hour. The sample was then diluted with ameasured amount of water and all particulate material was filtered out.A measured sample of the filtrate was then placed in a volumetric flask.Hydrogen peroxide was added to the flask to ensure all the titanium ionswere in the proper oxidation state for their concentration to bedetermined spectrophotometrically at 400 nm. The flask was then filledto volume with 10% sulfuric acid. The absorbance was measured vs. ablank containing the same amount of hydrogen peroxide as was added tothe sample in 10% sulfuric acid. The percent of titanium dioxide wasread from a calibration curve prepared from known standards.

High Resolution Electron Microscopy Procedures:

A combination of high resolution transmission EM (HREM) with atomicresolution and high resolution low voltage scanning EM (LVSEM) was usedto determine the microstructure, morphology, treatment layer thickness,uniformity and chemical composition.

Microstructure and high precision chemical compositional analyses on a(sub)nanometer scale were carried out by HREM and the associatedelectron stimulated energy dispersive X-ray compositional spectroscopy(EDX), respectively. A Philips CM200 field emission gun HREM/STEM,Philips CM20 HREM and a modified Philips CM30 environmental-HREMinstruments were used in the investigations, with an acceleratingvoltage of 200 kV (ref: P. L. Gai, DuPont: published in AdvancedMaterials, Vol. 10, p. 1259, 1998). All the EMs were equipped with X-rayspectrometers to analyze chemical composition.

The extent of treatment and treatment layer coverage observations weremade on all sides (including top and bottom surfaces) of the particlesusing standard sample tilting methods. For HREM, the pigment crystalswere oriented so that the desired crystal axes (e.g.<010>) were exactlyparallel to the electron beam. Primary magnifications were 100,000 to750,000.

A minimum sampling of 1000 particles having variable particle size anddimensionality was studied to represent an accurate measure of thefraction particles treated and the extent of the treatment surfacecoverage. HREM at atomic resolution was used to determine monolayercoatings as well as nanometers-scale coatings. Observations ofirregularity in treatment layers of partially coated and fully coatedparticles were carried out. Histograms were prepared according tostandard statistical methods were used to determine the fraction ofparticles where the treatment layer was full and complete at treatmentlayer thickness.

EXAMPLE

Titanium tetrachloride was pre-mixed with aluminum trichloride (chloridemix) and fed to the oxidation reactor. The amount of aluminumtrichloride in the mixture was sufficient to provide about 1 wt %aluminum oxide based on total solids formed in the oxidation reactor.

The chloride mix was evaporated and pre-heated to about 450° C. andintroduced into the reaction zone. Simultaneous with the introduction ofthe chloride mix, pre-heated oxygen (where the total excess oxygen wasabout 14 mole %) was continually introduced through a separate inletadjacent to the chloride mix inlet. Trace amounts of KCl dissolved inwater was added to the oxygen stream as disclosed in British Patent922,671 and U.S. Pat. No. 3,202,866. The Cl₂/TiCl₄ mole ratio in thefeed was 1:1.

Reaction temperature where the chloride mix contacted the oxygen wasabout 1550° C. Silicon tetrachloride was added as a dispersed liquiddown stream from where the chlorides mix and the oxygen streams wereinitially contacted at the point where approximately 1% of the titaniumtetrachloride remained unconverted. That is, more than 99% of thetitanium tetrachloride had been converted to titanium dioxide. Thesilicon tetrachloride was added in an amount sufficient to yield apigment having 1.2% of its total weight as silicon dioxide. Thetemperature at this point of addition was estimated to be approximately1100° C.

In the prior art, addition of the silicon tetrachloride at such a lowtemperature was considered impossible because of the concern thatsilicon tetrachloride would not have sufficient temperature to beconverted completely to the oxide. Although known reaction modelspredicted incomplete reaction of the silicon tetrachloride and oxygen atthe 1100° C.-reaction temperature, there was no unreacted silicontetrachloride in the product exiting the reactor.

The product pigment produced had the following properties. The fractionof the particles that were covered by the treatment layer was 97%. Thetreatment layer was uniformly deposited as is shown in FIGS. 1 a and 1b. The acid solubility of this pigment was about 21%. Acid solubility ofa typical wet treated durable grade is about 25%.

A second feature of the pigment produced was the low moisture content asmeasured using standard TGA methods. Losses of weight at 300° C. for thepresent product was about 0.6% and at 600° C. about 0.9% compared tocommercial product weight losses of 0.9% and 1.6%, respectively. Suchlow moisture content may be preferred for a pigment in plastic extrusionfilms.

The product pigment composition was 1.2% silicon dioxide and 1% aluminumoxide.

COMPARATIVE EXAMPLE

A control test was made according to U.S. Pat. No. 5,562,764 toGonzalez. The reaction conditions were the same as in the test above,except the addition of silicon tetrachloride was made at a point near 5feet downstream from the point where the oxygen and chlorides mixstreams were initially contacted and at a temperature of 1400 to 1500°C. At this point more than 5% of the titanium tetrachloride wasunreacted. The fraction of the particles that were covered by atreatment layer was only 16%. That is, about 84% of the pigmentparticles had surfaces that were not covered. The acid solubility ofthis pigment was about 35%.

In Gonzalez the addition of silicon tetrachloride was made to influencethe crystal form and pigment carbon black undertone of the pigmentproduced. The addition of silicon tetrachloride in the present inventionis made at a temperature less than 1200° C. and preferred to be nogreater than about 1100° C. At such temperatures the particles areessentially formed, and the silicon tetrachloride addition does notinfluence either crystal phase or pigment carbon black undertone.

1. A process for making durable titanium dioxide pigment by vapor phasedeposition of surface treatments on the titanium dioxide pigmentparticle surface, the process comprising the steps of: (a) reactingtitanium tetrachloride vapor and aluminum chloride and at least astoichiometric amount of oxygen in a plug flow reactor to form a productstream containing titanium dioxide particles; and (b) introducingsilicon tetrachloride into the reactor at one or more points downstreamof the point where the titanium tetrachloride and oxygen were contactedand where at least 97% of the titanium tetrachloride has been convertedto titanium dioxide.
 2. The process of claim 1 wherein the silicontetrachloride is introduced at a point where at least 98% of the.titanium tetrachloride has been converted to titanium dioxide.
 3. Theprocess of claim 1 wherein the silicon tetrachloride is introduced at apoint where at least 99% of the titanium tetrachloride has beenconverted to titanium dioxide.
 4. The process of claim 1 wherein thesilicon tetrachloride is introduced in an amount sufficient to provide asilica content of a surface treated titanium dioxide pigment of about atleast 1.2% by weight and the aluminum trichloride is added in an amountsufficient to provide an aluminum oxide content of a surface treatedpigment of at least about 1% by weight.
 5. The process of claim 1wherein steam or oxygen is introduced at a point downstream of the pointof introduction of silicon tetrachloride or wherein steam or oxygen areintroduced along with the silicon tetrachloride.
 6. A durable titaniumdioxide pigment wherein at least 85% of the pigment particles arecompletely covered by a uniform layer formed from a mixture of amorphousaluminum oxide and amorphous silicon dioxide, the pigment produced by:(a) reacting titanium tetrachloride vapor and aluminum chloride and atleast a stoichiometric amount of oxygen in a plug flow reactor to form aproduct stream containing titanium dioxide particles; and (b)introducing silicon tetrachloride into the reactor at one or more pointsdownstream of the point where the titanium tetrachloride and oxygen werecontacted and where at least 97% of the titanium tetrachloride has beenconverted to titanium dioxide.
 7. The pigment of claim 6 wherein thepercent of silicon dioxide is at least about 1.2% of the total weight ofthe pigment and the aluminum oxide is at least about 1% of the totalweight of the pigment.
 8. Durable titanium dioxide pigment particleshaving a surface treatment layer comprising aluminum oxide and silicondioxide wherein the at least 85% of the pigment particles are completelycovered by a uniform layer formed from a mixture of amorphous aluminumoxide and amorphous silicon dioxide and wherein the pigment particlesare free of debris.
 9. The titanium dioxide pigment of claim 8 wherein95% or more of the pigment particles are completely covered by a uniformlayer formed from a mixture of amorphous aluminum oxide and amorphoussilicon dioxide.
 10. A method to determine a point of introduction ofsilicon tetrachloride into a plug flow reactor for the oxidation of amixture of titanium tetrachloride and aluminum trichloride in a gascontaining at least a stoichiometric amount of oxygen to producetitanium dioxide particle having a thin, uniform and complete layer ofsurface oxides comprising a mixture of amorphous aluminum oxide andsilicon dioxide, the steps of the process comprising; (a) determiningthe temperature in the reactor where not more than 3% of the titaniumtetrachloride remains unreacted using $K = {{\frac{\begin{bmatrix}{{2\left( {{100\%} - u_{{TiC}\; l\; 4}} \right)} +} \\{\varphi \times 100\%}\end{bmatrix}^{2}}{u_{{TiCl}\; 4}\left( {\beta + u_{{TiCl}\; 4}} \right)}\mspace{14mu} {and}\mspace{14mu} T} < {\frac{20733}{{\ln \; K} + 6.391} - 273.15}}$where μ_(TiCl4)=unreacted TiCl4 (%) β=O2 (%) in excess of thestoichiometric amount φ=feed Cl2 mole ratio (mol/mol TiCl4), andT=temperature (C); and (b) introducing the silicon tetrachloride intothe reactor where the temperature is equal to or less than thetemperature calculated in step (a).
 11. A process for making durabletitanium dioxide pigment by vapor phase deposition of surface treatmentson the titanium dioxide pigment particle surface, the process comprisingthe steps of: (a) reacting titanium tetrachloride vapor, an oxygencontaining gas and aluminum chloride in a plug flow reactor to form aproduct stream containing titanium dioxide particles; and (b)introducing silicon tetrachloride into the reactor at one or more pointsdownstream of the point where the titanium tetrachloride and oxygen werecontacted and where the reaction temperature is no greater than about1200° C.